RNA therapeutics including antisense oligo nucleotides (ASOs), small-interfering RNA (siRNAs), synthetic mRNAs and genome editing RNA-protein complexes are emerging modalities for therapies targeting the human genomes at high specificity and great flexibility. ASOs and siRNAs have been widely used as the tools for gene knockdown in biomedical research. Their ability to silence any gene of interest offers a great potential for targeting disease-prevalent genes. Various chemical modifications or conjugations can be used to keep ASOs and siRNAs stable and enhance their binding specificity. Common methods for RNA transfection including nucleofection, lipofection and electroporation are only suitable for ex vivo delivery. Viral transduction and nanoparticles are often used for in vivo delivery of RNAs and DNAs however, these methods are usually ineffective, immunogenic and toxic.
One of the most recent breakthroughs in Science is a new technology for genome editing, the clustered regularly interspaced short palindromic repeats (CRISPR) method that enables robust and precise modifications of genomic DNA for a wide range of applications in research and medicine. CRISPR is an ideal tool for correction of genetic abnormalities in cancer as the system can be designed to target genomic DNA directly. A CRISPR system involves two main components: a Cas9 enzyme and a guide (gRNA). The gRNA contains a targeting sequence for DNA binding and a scaffold sequence for Cas9 binding. Cas9 nuclease is often used to “knockout” target genes hence it can be applied for deletion or suppression of oncogenes that are essential for cancer initiation or progression. Similar to ASOs and siRNAs, the CRISPR system offers a great flexibility in targeting any gene of interest hence, potential CRISPR based therapies can be designed based on the genetic mutation in individual patients. An advantage of the CRISPR system is its ability to completely ablate the expression of disease genes which can only be suppressed partially by RNA interference methods with ASOs or siRNAs. Furthermore, multiple gRNAs can be employed to suppress or activate multiple genes simultaneously, hence increasing the treatment efficacy and reducing resistance potentially caused by new mutations in the target genes. The applications of CRISPR technology have evolved very quickly from bench to bedside targeting different diseases. Clinical trials of CRISPR-mediated modification of T cells for cancer therapies have started in China and in the USA. Many other CRISPR-based therapies are under development. However, most of these therapies rely on ex vivo modification of the target cells or systemic delivery of the CRISPR system using virus or nanoparticles that can target very few cell types such as hepatocytes.
Acute myeloid leukemia (AML) is the most aggressive type of blood cancer that affects nearly 352,000 people per year with the 5-year prevalence of 1.5%. AML is characterized by the increase of myeloblasts in the peripheral blood (PB) and the bone marrow (BM). 30-40% AML patients (mostly under 60 years old) response well to chemotherapy and hematopoietic stem cell transplantation. However, the response rate is much lower in older patients as they cannot tolerate the toxicity of chemotherapy. Moreover, almost all the patients relapse after a certain time due to drug resistance. Hence, new treatment strategies are desirable to increase the response rate, reduce toxicity and combat drug resistance. Recent advances in genomics have provided better understanding of the genetic and epigenetic abnormalities in AML and suggest new specific therapeutic targets. RNA interference and genome editing methods are emerging as new approaches to target these abnormalities. However, delivery of RNAs to AML cells for gene therapies has proven challenging, especially for in vivo treatments. Common gene therapy delivery vehicles such as adeno-associated virus (AAV) and lipid nanoparticles (LNPs) are mostly ineffective or toxic in AML models.
Therefore, there is a desire to improve the delivery efficiency and reduce toxicity of gene therapies for cancer.